An Actinide\nComplex with a Nucleophilic Allenylidene\nLigand

The\nreaction of [Cp<sub>3</sub>Th(3,3-diphenylcyclopropenyl)] (Cp\n= η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>) with 1 equiv of\nlithium diisopropylamide (LDA) results in cyclopropenyl ring opening\nand formation of the thorium allenylidene complex, [Li(Et<sub>2</sub>O)<sub>2</sub>][Cp<sub>3</sub>Th(CCCPh<sub>2</sub>)] ([Li(Et<sub>2</sub>O)<sub>2</sub>][<b>1</b>]), in good yield. Additionally,\ndeprotonation of [Cp<sub>3</sub>Th(3,3-diphenylcyclopropenyl)] with\n1 equiv of LDA, in the presence of 12-crown-4 or 2.2.2-cryptand, results\nin the formation of discrete cation/anion pairs, [Li(12-crown-4)(THF)][Cp<sub>3</sub>Th(CCCPh<sub>2</sub>)] ([Li(12-crown-4)(THF)][<b>1</b>]) and [Li(2.2.2-cryptand)][Cp<sub>3</sub>Th(CCCPh<sub>2</sub>)]\n([Li(2.2.2-cryptand)][<b>1</b>]), respectively. Interestingly,\nthe complex [Li(Et<sub>2</sub>O)<sub>2</sub>][<b>1</b>] undergoes\ndimerization upon standing at room temperature, resulting in the formation\nof [Cp<sub>2</sub>Th(μ:η<sup>1</sup>:η<sup>3</sup>-CCCPh<sub>2</sub>)]<sub>2</sub> (<b>2</b>), via loss of LiCp.\nThe reaction of [Li(Et<sub>2</sub>O)<sub>2</sub>][<b>1</b>]\nwith MeI results in electrophilic attack at the C<sub>γ</sub> carbon atom, leading to the formation of a thorium acetylide complex,\n[Cp<sub>3</sub>Th(CCC(Me)Ph<sub>2</sub>)] (<b>3</b>),\nwhich can be isolated in 83% yield upon workup, whereas the reaction\nof [Li(Et<sub>2</sub>O)<sub>2</sub>][<b>1</b>] with benzophenone\nresults in the formation of 1,1,4,4-tetraphenylbutatriene (<b>4</b>) in 99% yield, according to integration against an internal standard.\nDensity functional theory (DFT) calculations performed on [<b>1</b>]<sup>−</sup> and <b>2</b> reveal significant electron\ndelocalization across the allenylidene ligand. Additionally, calculations\nof the <sup>13</sup>C NMR chemical shifts for the C<sub>α</sub>, C<sub>β</sub>, and C<sub>γ</sub> nuclei of the allenylidene\nligand were in good agreement with the experimental shifts. The calculations\nreveal modest deshielding induced by spin–orbital effects originating\nat Th due to the involvement of the 5f orbitals in the Th–C\nbonds. According to a DFT analysis, the cyclopropenyl ring-opening\nreaction proceeds via [Cp<sub>3</sub>Th(η<sup>1</sup>-3,3-Ph<sub>2</sub>-<i>cyclo</i>-C<sub>3</sub>)]<sup>−</sup> (<b>IM</b>), which features a carbanion character at C<sub>β</sub>.